Thin film processing method and thin film processing apparatus

ABSTRACT

A thin film processing method for processing the thin film by irradiating optical beam to the thin film. A unit of the irradiation of the optical beam includes a first and a second optical pulse irradiation to the thin film and the unit of the irradiation is carried out repeatedly to process the thin film. The first and the second optical pulse have pulse waveforms different from each other. Preferably, a unit of the irradiation of the optical beam includes the a first optical pulse irradiated to the thin film and a second optical pulse irradiated to the thin film starting substantially simultaneous with the first optical pulse irradiation. In this case, the relationship between the first and the second optical pulse satisfies (the pulse width of the first optical pulse)&lt;(the optical pulse of the second optical pulse) and (the irradiation intensity of the first optical pulse)≧(the irradiation intensity of the second optical pulse). A silicon thin film with a small trap state density can be formed by the optical irradiation.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a system for the formation of a siliconthin film and a good-quality semiconductor-insulating film interface.Such silicon thin films are used for crystalline silicon thin filmtransistors, and such semiconductor-insulating film interfaces areemployed for field effect transistors. The invention also relates to asemiconductor thin film forming system by the pulsed laser exposuremethod. In addition, the invention relates to a system for themanufacture of driving elements or driving circuits composed of thesemiconductor thin films or field effect thin film transistors fordisplays and sensors, for example.

[0003] 2. Description of the Related Art

[0004] Typical processes for the formation of a thin film transistor(TFT) on a glass substrate are a hydrogenated amorphous silicon TFTprocess and a polycrystalline silicon TFT process. In the formerprocess, the maximum temperature in a manufacture process is about 300°C., and the carrier mobility is about 1 cm²/Vsec. Such a hydrogenatedamorphous silicon TFT formed by the former process is used as aswitching transistor of each pixel in an active matrix (AM) liquidcrystal display (LCD) and is driven by a driver integrated circuit (IC,an LSI formed on a single crystal silicon substrate) arranged on theperiphery of a screen. Each of the pixels of this system includes anindividual switching element TFT, and this system can yield a betterimage quality with a less crosstalk than a passive matrix LCD. In such apassive matrix LCD, an electric signal for driving the liquid crystal issupplied from a peripheral driver circuit. In contrast, the latterpolycrystalline silicon TFT process can yield a carrier mobility of 30to 100 cm²/Vsec by, for example, employing a quartz substrate andperforming a process at high temperatures of about 1000° C. as in themanufacture of LSIs. For example, when this process is applied to aliquid crystal display manufacture, such a high carrier mobility canyield a peripheral driver circuit on the same glass substrateconcurrently with the formation of pixel TFTs for driving individualpixels. This process is therefore advantageous to minimize manufactureprocess costs and to downsize the resulting products. If the productshould be miniaturized and should have a higher definition, a connectionpitch between an AM-LCD substrate and a peripheral driver integratedcircuit must be decreased. A conventional tab connection method or wirebonding method cannot significantly provide such a decreased connectionpitch. However, if a process at high temperatures as in the above caseis employed in the polycrystalline silicon TFT process, low softeningpoint glasses cannot be employed. Such low softening point glasses canbe employed in the hydrogenated amorphous silicon TFT process and areavailable at low costs. The process temperature in the polycrystallinesilicon TFT process should be therefore decreased, and techniques forthe formation of polycrystalline silicon films at low temperatures havebeen developed by utilizing a laser-induced crystallization technique.

[0005] Such a laser-induced crystallization is generally performed by apulse laser irradiator having a configuration shown in FIG. 15. A laserlight supplied from a pulse laser source 1101 reaches a silicon thinfilm 1107, a work, on a glass substrate 1108 via an optical path 1106.The optical path 1106 is specified by a group of optic devices includingmirrors 1102, 1103, and 1105, and a beam homogenizer 1104. The beamhomogenizer 1104 is arranged to uniformize spatial intensities of laserbeams. Generally, because the irradiation area is smaller than that ofthe glass substrate 1108, the glass substrate on an X-Y stage 1109 ismoved to irradiate an optional position on the substrate with a laserbeam. The laser irradiation can be also performed by moving the opticdevice group or moving the optic device group and the stage incombination. Laser irradiation may also be carried out in a vacuum or inthe high purity gas atmosphere within the vacuum chamber. Whennecessary, a cassette 1110 having the glass substrate with silicon thinfilm and a substrate carrier mechanism 1111 are provided formechanically separating and accommodating the substrate between thecassette and the stage.

[0006] Japanese Patent Publication (JP-B) No. 7-118443 discloses atechnique of irradiating an amorphous silicon thin film on an amorphoussubstrate with a short wavelength pulse laser light. This technique cancrystallize an amorphous silicon while keeping the overall substratefrom high temperatures, and can produce semiconductor elements orsemiconductor integrated circuits on large substrates available at lowcosts. Such large substrates are required in liquid crystal displays,and such substrate available at low costs may be glasses, for example.However, as is described in the above publication, the crystallizationof an amorphous silicon thin film by action of a short wavelength laserlight requires an irradiation intensity of about 50 to 500 mJ/cm².However, the maximum emission output of a conventionally available pulselaser irradiator is at most about 1 J/pulse, and an area to beirradiated by a single irradiation is at most about 2 to 20 cm², by asimple conversion. For example, if the overall of a 47 cm×37 cmsubstrate should be crystallized by action of laser, at least 87 to 870points of the substrate must be irradiated with a laser light. Likewise,the number of points to be irradiated with a laser light increases withan increasing size of the substrate, for example, as in a 1 m×1 msubstrate. Such a laser-induced crystallization is generally performedby a pulse laser irradiator having a configuration shown in FIG. 15.

[0007] To form uniform thin film semiconductor elements on a largesubstrate by the above technique, an effective process is known asdisclosed in Japanese Unexamined Patent Publication (JP-A) No. 5-211167(Japanese Patent Application No. 3-315863). The process includes thesteps of dividing the elements to portions smaller than the beam size ofthe laser and repeating a combination of irradiation with several pulsesand movement of the area to be irradiated by step-and-repeat drawingmethod. In the process, the lasing and the movement of a stage (i.e.,the movement of a substrate or laser beam) are alternatively performed,as shown in FIG. 16(2). However, even according to this process, thevariation of lasing intensity exceeds ±5% to ±10% when the irradiationprocedure is repeated at a density of about 1 pulse per irradiatedportion to 20 pulses per irradiated portion using a currently availablepulse laser irradiator with a uniformity of lasing intensity of ±5% to±10% (in continuous lasing). The resulting polycrystalline silicon thinfilm and polycrystalline silicon thin film transistor cannot thereforehave satisfactorily uniform characteristics. Particularly, thegeneration of a strong or weak light caused by an unstable discharge atearly stages of lasing significantly invites such heterogeneouscharacteristics. This phenomena is called as spiking. As a possiblesolution to the spiking, a process of controlling an applied voltage ina subsequent lasing with reference to the results of integratedstrengths can be employed. However, according to this process, a weaklight is rather oscillated even though the formation of spiking isinhibited. Specifically, when irradiation periods and non-lasing periodsalternatively succeed, the intensity of a first irradiated pulse in eachirradiation period is most unstable and is varied, as shown in FIG. 17.In addition, the history of irradiation intensity differs from point topoint to be irradiated. The resulting transistor element and thin filmintegrated circuit cannot have a significant uniformity in the substrateplane.

[0008] To avoid such a spiking, a process is known to start lasing priorto the initiation of irradiation to an area for the formation ofelement, as shown in FIG. 16(2). However, this technique shown in FIG.16(2) cannot be applied to a process of intermittently repeating thelasing and the movement of stage. To avoid these problems, a process isproposed in Japanese Unexamined Patent Publication (JP-A) No. 5-90191.The process includes the steps of allowing a pulse laser source tocontinuously oscillate and inhibiting irradiation of a substrate withthe laser light by an optic shielding system during the movement of thestage. Specifically, as shown in FIG. 16(3), a laser is continuouslyoscillated at a predetermined frequency, and the movement of stage to atarget irradiation position is brought into synchronism with theshielding of an optic path. By this configuration, a laser beam with astable intensity can be applied to a target irradiation position.However, although this process can stably irradiate the substrate with alaser beam, the process also yields increased excess lasing that doesnot serve to the formation of a polycrystalline silicon thin film. Theproductivity is decreased from the viewpoint of the life of an expensivelaser source and an excited gas, and the production efficiency of thepolycrystalline silicon thin film is deteriorated with respect to powerrequired for lasing. The production costs are therefore increased. Whena substrate to be exposed to laser is irradiated with an excessivelystrong light as compared with a target intensity, the substrate will bedamaged. Such an excessively strong light is induced by an irregularirradiation intensity. In LCDs and other imaging devices, a lightpassing through the substrate scatters in an area where the substrate isdamaged, and the quality of image is deteriorated.

[0009] A process is known for the laser irradiation. In this process, aplurality of pulses are applied while the irradiation of each pulse isretarded. This process is disclosed by Ryoichi Ishihara et al. in“Effects of light pulse duration on excimer laser crystallizationcharacteristics of silicon thin films”, Japanese Journal of AppliedPhysics, vol. 34, No. 4A, (1995), pp 1759 (hereinafter called asReference). According to this reference, the crystallizationsolidification rate of a molten silicon in a laser recrystallizationprocess is 1 m/sec or more. To achieve a satisfactory growth ofcrystals, the solidification rate must be reduced. By applying a secondlaser pulse immediately after the completion of solidification, thesecond irradiation of laser pulse can yield a recrystallization processwith a less solidification rate. In viewing a temperature change (atime-hysteresis curve) of silicon as shown in FIG. 18, the temperatureof silicon increases with the irradiation of laser energy, for example,as a pulse with an intensity shown in FIG. 19. When a starting materialis an amorphous silicon (a-Si), the temperature further increases afterthe melting point of a-Si, and when the supplied energy becomes lessthan the energy required for increasing the temperature, the materialbegins to undergo cooling. At the solidifying point of a crystalline Si,the solidification proceeds for a solidification time and thencompletes, and the material is cooled to an atmospheric temperature.Provided that the solidification of silicon proceeds in a thicknessdirection from an interface between silicon and the substrate, anaverage solidification rate is calculated according to the followingequation.

Average solidification rate=(Thickness of silicon)/(Solidification time)

[0010] Specifically, if the thickness of silicon is constant, thesolidification time is effectively prolonged to reduce thesolidification rate. If the process maintains ideal conditions onthermal equilibrium, the solidification time can be prolonged byincreasing an ideally supplied energy, i.e., a laser irradiation energy.However, as pointed out in the above reference, such an increasedirradiation energy invites the resulting film to become amorphous ormicrocrystalline. In an actual melting and recrystallization process,the temperature does not change in an ideal manner as shown in FIG. 18,and the material undergoes overheating when heated and undergoessupercooling when cooled, and attains a stable condition. Particularly,when the cooling rate in cooling procedure is extremely large and thematerial undergoes an excessive supercooling, the material is notcrystallized at around its solidification point, and becomes anamorphous solid due to quenching and rapid solidification. Under someconditions, thin films are converted not into amorphous but intomicrocrystals, as shown in the above-mentioned Reference.

[0011] Accordingly, an object of the invention is to provide a processfor forming a semiconductor thin film with a less trap state density bylight irradiation and to provide a process and system for applying theabove process to large substrates with a high reproducibility.

[0012] Another object of the invention is to provide a means for forminga satisfactory gate insulating film on the semiconductor thin film ofgood quality and to provide a system for producing a field effecttransistor having a satisfactory semiconductor-insulating filminterface, i.e., satisfactory properties.

SUMMARY OF THE INVENTION

[0013] (1) According to the present invention, there is provided a thinfilm processing method for processing the thin film by irradiating theoptical beam to the thin film, wherein

[0014] one set of irradiation of the optical beam includes a first and asecond optical pulse irradiated to the thin film, and

[0015] each of the first and the second optical pulse has differentpulse waveforms.

[0016] (2) According to the present invention, there is provided a thinfilm processing method wherein

[0017] the one set of irradiation includes the first optical pulseirradiation to the thin film and the second optical pulse irradiation tothe thin film which substantially starts simultaneously with theirradiation of the first pulse.

[0018] (3) According to the present invention, there is provided a thinfilm processing method as described in (2), wherein

[0019] the relationship between the first and the second pulse satisfies

(the pulse width of the first optical pulse)<(the pulse width of thesecond optical pulse),

(the irradiation intensity of the first optical pulse)≧(the irradiationintensity of the second optical pulse).

[0020] (4) According to the present invention, there is provided a thinfilm processing method as described in (1), wherein

[0021] the one set of irradiation includes the first optical pulseirradiation to the thin film and the second optical pulse irradiation tothe thin film which substantially starts with a delay to the firstoptical pulse irradiation.

[0022] (5) According to the present invention, there is provided a thinfilm processing method as described in (4), wherein

[0023] the relationship between the first and the second pulse satisfies

(the pulse width of the first optical pulse)<(the pulse width of thesecond optical pulse).

[0024] (6) According to the present invention, there is provided a thinfilm processing method as described in (5), wherein

[0025] the relationship between the first and the second pulse satisfies

(the irradiation intensity of the first optical pulse)≧(the irradiationintensity of the second optical pulse).

[0026] (7) According to the present invention, there is provided anapparatus for processing thin film by irradiating the optical pulse tothe thin film, the thin film processing apparatus comprises

[0027] a first and a second pulse optical source for producing a firstand a second optical pulse, respectively, wherein the first and thesecond optical pulse has different pulse waveforms, and

[0028] the one set of irradiation of the optical beam includes theirradiation for the first and the second optical pulse to the thin film,wherein the set of irradiation is carried out repeatedly.

[0029] (8) According to the present invention, there is provided a thinfilm processing apparatus as described in (7), wherein

[0030] the one set of irradiation includes the first optical pulseirradiation to the thin film and the second optical pulse irradiation tothe thin film which substantially starts simultaneously with theirradiation of the first pulse.

[0031] (9) According to the present invention, there is provided a thinfilm processing apparatus as described in (8), wherein

[0032] the relationship between the first and the second pulse satisfies

(the pulse width of the first optical pulse)<(the pulse width of thesecond optical pulse),

(the irradiation intensity of the first optical pulse)≧(the irradiationintensity of the second optical pulse).

[0033] (10) According to the present invention, there is provided a thinfilm processing apparatus as described in (7), wherein

[0034] the one set of irradiation includes the first optical pulseirradiation to the thin film and the second optical pulse irradiation tothe thin film which substantially starts with a delay to the firstoptical pulse irradiation.

[0035] (11) According to the present invention, there is provided a thinfilm processing method as described in (10), wherein

[0036] the relationship between the first and the second pulse satisfies

(the pulse width of the first optical pulse)<(the pulse width of thesecond optical pulse).

[0037] (12) According to the present invention, there is provided a thinfilm processing apparatus as described in (11), wherein

[0038] the relationship between the first and the second pulse satisfies

(the irradiation intensity of the first optical pulse)≧(the irradiationintensity of the second optical pulse).

[0039]FIG. 11 shows the relationship of the maximum cooling rate(Cooling rate, K/sec) obtained by mathematical calculation with thethreshold irradiation intensity between crystallization andmicrocrystallization. In this case, a 75-nm silicon thin film isirradiated with an excimer laser with a wavelength of 308 nm, and thethreshold is obtained by a scanning electron microscopic (SEM)observation of the silicon thin film after laser irradiation. FIG. 19shows an emission pulse shape of the laser used in the experiment. Thispulse shap exhibits a long emission time 5 tim s or more that of arectangular pulse with a pulse width of 21.4 nsec described in therelevant Reference. Even a single pulse irradiation with the pulse shapein question is therefore expected to reduce the solidification rate asdescribed in the Reference. FIG. 12 shows a calculated temperature-timecurve of silicon in laser recrystallization using the pulse shape inquestion. Specifically, FIG. 12 shows the temperature change of asilicon thin film 75 nm thick on a SiO₂ substrate when an XeCl laser(having a wavelength of 308 nm) is applied at an irradiation intensityof 450 mJ/cm². About 60 nsec into the irradiation, a second emissionpeak nearly completes, and the temperature attains the maximum and thenturns to decrease. (In this connection, in the mathematical calculation,a melting-solidification point of amorphous silicon is employed as themelting-solidification point, and the behavior of the material round thesolidification point differs from that in actual case. Particularly whena crystallized film is obtained, the crystallization completes at thesolidification point of the crystalline silicon.) The curve has a largegradient upon the initiation of cooling, but has a very small gradientat about 100 nsec, i.e., at a third emission peak. At elapsed time of120 nsec, the light emission completely ceases, and the silicon is thensolidified through another rapid cooling process. Generally, when aliquid is solidified through “quenching” which is greatly out of athermal equilibrium process, a sufficiently long solidification timecannot be obtained to form a crystal structure, and the resulting solidis amorphous (non-crystal). The maximum cooling rate was estimated froma temperature-time curve of silicon as shown in FIG. 12. FIG. 11 showsthe estimated maximum cooling rates after the completion of lightemission with respect to individual irradiation intensities. The figureshows that the cooling rate increases with an increasing irradiationintensity. Separately, the structure of the silicon thin film afterlaser irradiation was observed with a scanning electron microscope. As aresult, as shown in FIG. 13, the grain size once increased with anincreasing irradiation intensity, but microcrystallization was observedat a set irradiation intensity of about 470 mJ/cm². When the film wasirradiated with three laser pulses, the grain size markedly increasedeven at a set irradiation intensity of about 470 mJ/cm², while amicrocrystallized region partially remained (FIG. 13). This largeincrease of the grain size differs from the behavior of the grain sizein the one-pulse irradiation. In this connection, an actual irradiationintensity is 5% to 10% higher than the set level, typically in initialseveral pulses of excimer laser. The threshold intensity at whichmicrocrystallization occurs can be therefore estimated as about 500mJ/cm². Based on these results, the cooling rate at 500 mJ/cm² as shownin FIG. 11 is estimated, and microcrystallization is found to occur at acooling rate of about 1.6×10¹⁰° C./sec or more. When the film to beirradiated is an a-Si film, the microcrystallization occurs at anirradiation intensity of about 500 mJ/cm² or more. Likewise, when thefilm to be irradiated is a poly-Si film, the microcrystallization mayoccur at an irradiation intensity about 30 mJ/cm² higher than that inthe a-Si at the same cooling rate of about 1.6×10¹⁰° C./sec. Bycontrolling the cooling rate to 1.6×10¹⁰° C./sec or less, therefore, theresulting crystal can be kept from becoming microcrystalline oramorphous and can satisfactorily grow.

[0040] Next, the case where a delayed second laser light is irradiatedwith a delay relative to a first laser light. As is described above, alaser light at a late light emission stage suppresses the increase ofthe cooling rate, and the cooling rate after the completion of lightemission controls the crystallization. The last supplied energy issupposed to initialize precedent cooling processes. Specifically, bysupplying an additional energy, a precedent cooling process is onceinitialized and a solidification process is repeated again, even if thecrystal becomes amorphous or microcrystalline in the precedent coolingprocess. This is provably because the interval of light irradiation isvery short of the order of nanoseconds, and loss of the energy bythermal conduction to the substrate and radiation to the atmosphere issmall. The energy previously supplied therefore remains nearly asintact. In this assumption, a long time interval sufficient to dissipateheat is not considered. Accordingly, by controlling th cooling rateafter the completion of a second heating by the additionally suppliedenergy, the crystal is expected to grow satisfactorily. As shown in FIG.14, the cooling rate is controlled to a desired level by controlling thedelay time of the second laser irradiation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 shows an optical pulse waveform for use in describing theembodiment according to the present invention.

[0042]FIG. 2 is a diagram showing and embodiment (the overallconfiguration) of an embodiment of the invented exposure system.

[0043]FIG. 3 is a diagram showing an embodiment (aligning process) ofthe invented exposure system.

[0044]FIG. 4 are diagrams showing an embodiment (mask projectionprocess) of the invented exposure system.

[0045]FIG. 5 are diagrams showing embodiments (control procedures) ofthe invented exposure system.

[0046]FIG. 6 is a side sectional view showing the invented exposuresystem, transfer chamber, and plasma-enhanced CVD chamber.

[0047]FIG. 7 is a top view of the invented composite system including,for example, an exposure system, transfer chamber, and plasma-enhancedCVD chamber.

[0048]FIG. 8 shows sectional views showing the invented process forproducing TFT.

[0049]FIG. 9 shows sectional views showing the invented process forproducing TFT using alignment mark.

[0050]FIG. 10 shows sectional views showing the invented process forproducing TFT including the formation of an alignment mark.

[0051]FIG. 11 is a diagram showing the relationship between theirradiation intensity and the cooling rate, and the cooling rate atwhich th film becomes amorphous.

[0052]FIG. 12 is an illustrative diagram of calculated temperaturechanges of a silicon thin film.

[0053]FIG. 13 is a diagram showing crystal forms of silicon thin filmscorresponding to individual irradiation intensities.

[0054]FIG. 14 is a diagram showing the maximum cooling rate after thesupply of a second pulse, and the cooling rate around the solidificationpoint.

[0055]FIG. 15 is a schematic view of a conventional excimer laserannealing apparatus.

[0056]FIG. 16 is a timing chart showing conventional and inventedoperation procedures of laser annealing.

[0057]FIG. 17 is a diagram showing the pulse to pulse stability of laserpulse intensities.

[0058]FIG. 18 is a diagram showing an illustrative temperature change ofa silicon film.

[0059]FIG. 19 is a diagram showing an illustrative laser pulse shape.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0060] The embodiments of the invention will now be illustrated indetail with reference to the drawings.

[0061]FIG. 1 illustrates an example of the embodiment of the presentinvention. Each of the oscillation start timings is depicted as theabscissa axis while the irradiation energy is depicted as the regionbound by the pulse line. FIG. 1(a) shows an example where a first pulselaser and a second pulse laser are oscillated at the simultaneoustiming. The time interval required between the supply of the triggersignal for controlling the oscillation and the actual start of theoscillation often depends upon the construction of the laser apparatus.Therefore, the “trigger oscillation” time is predetermined so that theirradiation can b simultaneousely carried out. Since th emission time ofthe second pulse is longer than that of the first pulse, thegradual-cooling effect becomes higher during the melting and solidifyingprocess. Moreover, during the melting process by the first pulse, withthe advantageous of the initial region processed by the second pulse,the larger area can be melted at once such that the processing speedwill become much faster.

[0062]FIG. 1(b) shows an example of the state in which the second pulseis supplied with a delay to the oscillation of the first pulse. Thestate is much more preferable when the first pulse utilizes the opticalsource with a small pulse width. In case where the melting andrecrystallizing process is performed only by the first pulse, thegradual-cooling is carried out because the amount of heat providedtogether with the increase of the irradiation intensity increases.However, as shown in FIG. 11, when the maximam cooling rate increaseswithin an extremely short time interval during the laser irradiationprocess and exceeds the critical cooling rate, the solidifying processdeviates the ideal thermal equilibrium state. As a result, themicrocrystalline or amorphous crystal are to be observed in the filmthus obtained. The above-mentioned maximum cooling rate is observedimmediately after the peak of the irradiation pulse has been irradiated.At this stage, it is possible to recover the melting state by supplyingthe additive energy before the completion of the cooling process. Byirradiating, as the means to supply the additive energy, the pulse thathas a long pulse width and a small peak intensity, it is possible toprevent the deviation to the non-equilibrium state and to realize thegradual-cooling process. It is required to predetermine by theexperiment the delay time of the second pulse because it variesdepending on the intensity and the waveform of the first pulse. In thepresent embodiment, the preferable delay time is about 50-200 nsec.Since the pulse width used as the first pulse was about 120 nsec, underthe condition where the delay time exceeds 120 nsec, the second pulse iscontrolled to be irradiated after the completion of the emission of thefirst pulse as shown in FIG. 1(c).

[0063]FIG. 2 shows an embodiment of the invention. Pulsed ultraviolet(UV) beams are supplied from a first excimer laser EL1 and a secondexcimer laser EL2 and are introduced via mirrors opt3 and otp3′ andlenses opt4 to a homogenizer opt20′. The intensity profile of the beamis adjusted in the homogenizer so as to attain a target uniformity in aphoto mask opt21, for example, an in-plane distribution of ±5%.(Original beams supplied from the excimer lasers may have an intensityprofile or total energy which vanes pulse to pulse. The system thereforepreferably includes a mechanism for adjusting the spatial intensitydistribution and pulse-to-pulse intensity variation on the photo mask toachieve a higher uniformity. The homogenizer generally includes afly-eye lens or a cylindrical lens.) The patterned light formed by thephoto mask is applied via a reduction projection optical system opt23′and a laser inlet window W0 onto a substrate sub0 placed in a vacuumchamber C0. The substrate is mounted on a substrate stage S0, and atarget region, for example, a pattern transfer region ex0, can beexposed to the patterned light by operating the substrate stage. In FIG.2, the reduction projecting optical system is illustrated, but thesystem can include a 1:1 projecting optical system or an enlargementprojecting optical system. An optional region on the substrate isirradiated with the patterned light by moving the substrate stage in X-Ydirection in the figure. The photo mask is mounted on a mask stage (notshown), and the beam to be applied on the substrate can be controlledalso by moving the photo mask within a region capable of exposing.

[0064] To apply a target patterned light onto the substrate underdesired conditions, a mechanism is required. An illustrative mechanismwill now be described. As an optical axis should be delicately andprecisely adjusted, in the following example, the optical axis is onceadjusted and then fixed, and the position of the substrate is adjustedto control the irradiation. For adjusting the position of the irradiatedsurface of the substrat relative to the optical axis, the position ofthe surface in a direction of the focus (Z direction) and theverticality relative to the optical axis must be corrected. Of the θxytilt correction direction, θxz tilt correction direction, θyz tiltcorrection direction, X exposure region moving direction, Y exposureregion moving direction, and Z focusing direction in the figure, theverticality relative to the optical axis is corrected by adjusting inthe θxy tilt correction direction, θxz tilt correction direction, andθyz tilt correction direction. The position of the irradiated surface ofthe substrate is controlled to an appropriate position according to thefocal depth of the optical system by adjusting the Z focusing direction.

[0065]FIG. 3 is an illustrative side sectional view of the adjustmentand alignment mechanism of the substrate. The photo mask opt21, thereduction projection optical system opt23′, and the laser inlet windowW0 are arranged with respect to an exposure axis L0, as shown in thefigure. The substrate sub0 placed in a vacuum chamber C0 is mounted on aheater H0 with a substrate adhesion mechanism, and asubstrate-XYZθxyθxzθyz-stage S0′. In this embodiment, a vacuum chamberis used, but an actual light irradiation should be preferably performedin an atmosphere of, for example, an inert gas, hydrogen gas, oxygengas, or nitrogen gas. The inside of the chamber is once evacuated and isthen replaced with the above-mentioned gas. The pressure in the chambermay be around atmospheric (barometric) pressure. By using a heater witha substrate adhesion mechanism, the substrate can be heated at atemperature of from room temperature to about 400° C. in lightirradiation procedure. When the inside pressure is set around barometricpressure, the substrate can be adhered to the heater through a vacuumchucking mechanism. Accordingly, the misalignment of the substrate canbe inhibited even if the substrate stage moves in the chamber, and thesupplied substrate can be surely fixed to the substrate stage even ifthe substrate has some warp or bending. In addition, the shift of thefocal depth due to heat-induced warp or bending can be minimized.

[0066] Laser interferometers i1 and i2 make alignment of the substrateand a measurement of the position of the substrate in Z direction, via alength measuring window W-i and a length measuring mirror opt-i. Toalign the substrate, the position of an alignment mark on the substrateis determined with an off-axis microscope m0, a microscope light sourceLm, and a microscope element opt-m. A target exposure position can bedetermined using information about the substrate position obtained fromthe laser interferometer system. In FIG. 3, the off-axis alignment isillustrated, but the invented system can also employ through-the-lensalignment or through-the-mask (through-the-reticle) alignment. In themeasurement, measurement errors can be averaged by making measurementsfrom plural measuring points and determining a linear coordinate basedon the measured data through the least square method.

[0067] FIGS. 4(A) to 4(C) show the relationship between a mask patternand an alignment mark. The mask includes a mask (non-exposure area)mask1 and a mask (exposure area) mask2. For example, when an excimerlaser is used as the light source, a film that absorbs and reflectsultraviolet radiation is formed on a quartz substrate. The ultravioletradiation passes through such a quartz substrate. The film is formedfrom, for example, aluminium, chromium, tungsten, or other metals, or isa dielectric multilayer film, and is then patterned by photolithographyand etching processes to yield the mask. According to a target patternon the mask (indicated by the white areas in FIG. 4(A), a silicon filmis exposed to yield exposed Si portions (Si2) in a non-exposed Si (Si1)as shown in FIGS. 4(B) and 4(C). Where necessary, alignment andadjustment is conducted to make a mark on the mask mark1 agree with amark on the substrate mark2 prior to exposure. A predetermined anddesigned region on the silicon thin film can be therefore exposed. Inthe thin film transistor forming process using a silicon thin film, ifthe exposure process is a first process requiring the alignment (i.e.,no alignment mark is formed prior to the exposure process), an exposedmark mark3 should be preferably formed by exposure concurrently in theexposure process of the silicon thin film. By this procedure, analignment mark can be formed using an optical color difference betweena-Si and crystallin Si. By performing, for example, photolithography ina successive process with reference to the above alignment mark,transistors and other desired mechanisms and functions can be formed intarget regions which are exposed and modified. Subsequent to theexposure process, an Si oxide film is formed on the silicon thin filmand a target region of the silicon film is removed by etching. FIG. 4(C)show the state just mentioned above. A removed Si region (Si3) is aregion where the laminated silicon film and Si oxide film are removed byetching. In this configuration, Si oxide films (Si4 and Si5) arelaminated on the non-exposed Si (Si1) and the exposed Si (Si2). Byforming island structures including a silicon film covered with an oxidefilm as stated above, desired channel-source-drain regions of a thinfilm transistor or alignment marks necessary for successive processescan be formed. In such a transistor, elements are separated from oneanother.

[0068] FIGS. 5(1) and (2) are timing charts of essential controlprocedures. In the illustrative control procedure (1), the substrate ismoved to a target exposure position by operating the substrate stage.Next, the exposure position is accurately adjusted by focusing oralignment operation. In this procedure, the exposure position isadjusted to achieve a target predetermined accuracy of error of, forexample, about 0.1 μm to 100 μm. On completion of this operation, thesubstrate is irradiated with light. On completion of series of theseoperations, the substrate is moved to a successive exposure position. Oncompletion of irradiation of all the necessary regions on the substrate,the substrate is replaced with a new one, and the second substrate to betreated is subjected to a series of the predetermined operations. In thecontrol procedure (2), the substrate is moved to a target exposureposition by operating the substrate stage. Next, the exposure positionis accurately adjusted by focusing or alignment operation. In thisprocedure, the exposure position is adjusted to achieve a targetpredetermined accuracy of error of, for example, about 0.1 μm to 100 μm.On completion of this operation, the mask stage starts to operat. In theillustration, the substrate is irradiated with light after theinitiation of the mask stage operation to avoid variation of movingsteps during startup. Naturally, a region at a distance from thealignment position is to be exposed due to the movement of the stage,and an offset corresponding to the shift must be previously considered.To avoid unstable operations, the light source may be operated prior tothe light irradiation to the substrate, and the substrate may beirradiated with light by opening, for example, a shutter. Particularly,when an excimer laser is employed as the light source and lasing periodsand suspension periods are repeated in turn, several ten pulses emittedat early stages are known to be particularly unstable. To avoidirradiation with these unstable laser pulses, the beams can beintercepted according to the operation of the mask stage. On completionof irradiation of all the necessary regions on the substrate, thesubstrate is replaced with a new one, and the second substrate to betreated is subjected to a series of the predetermined operations.

[0069] In this connection, an a-Si thin film 75 nm thick was scannedwith a 1 mm×50 μm beam at a 0.5-μm pitch in a minor axis direction. Whenthe scanning (irradiation) was performed using one light source at alaser irradiation intensity of the irradiated surface of 470 mJ/cm², acontinuous single-crystal silicon thin film in the scanning directionwas obtained. In addition, a beam from a second light source was appliedwith a delay time of 100 nsec to yield a laser irradiation intensity ofthe irradiated surface of 150 mJ/cm², a continuous single-crystalsilicon thin film in the scanning direction was obtained, even at ascanning pitch of 1.0 μm. The trap state density in the crystallizedsilicon film was less than 10¹² cm⁻².

[0070]FIG. 6 is a side sectional view of an embodiment of the inventedsemiconductor thin film forming system. The system includes aplasma-enhanced CVD chamber C2, a laser irradiation chamber C5, and asubstrate transfer chamber C7. In the system, the substrate can betransferred via gate valves GV2 and GV5 without exposing to anatmosphere outside the system. The transfer can be performed in vacuo orin an atmosphere of an inert gas, nitrogen gas, hydrogen gas or oxygengas, in high vacuum, under reduced pressure or under pressure. In thelaser irradiation chamber, the substrate is placed on a substrate stageS5 with the aid of a chucking mechanism. The substrate stage S5 can beheated to about 400° C. In the plasma-enhanced CVD chamber, thesubstrate is placed on a substrate holder S2. The substrate holder S2can be heated to about 400° C. The figure illustrates the followingstate. A silicon thin film Si1 is formed on a glass substrate Sub0, andthe substrate is then brought into the laser irradiation chamber. Thesurface silicon thin film is modified into a crystalline silicon thinfilm Si2 by laser irradiation, and the substrate is then transferred tothe plasma-enhanced CVD chamber.

[0071] Laser beams are brought into the laser irradiation chamber in thefollowing manner. The laser beams are supplied from an excimer laser 1(EL1) and an excimer laser 2 (EL2), pass through a first beam line L1and a second beam line L2 and a laser composing optical system opt1, amirror opt11, a transmissive mirror opt12, a laser irradiation opticalsystem opt2, a homogenizer opt20, a photo mask opt21 mounted and fixedon a photo mask stage opt22, a projection optical system opt23, and alaser inlet window W1, and reach the substrate surface. In this figure,two excimer lasers are illustrated, but an optional number (one or more)of light sources can be employed in the system. The light source is notlimited to the excimer laser and includes, for example, carbon gaslaser, yttrium-aluminum-garnet (YAG) laser, and other pulse lasers. Inaddition, laser pulses Can be made and applied onto the substrate byusing argon laser or another continuous wave (CW) light source and ahigh speed shutter.

[0072] In the plasma-enhanced CVD chamber, a radio frequency (RF)electrode D1 and a plasma confinement electrode D3 constitute a plasmagenerating region D2 at a position at a distance from a region where thesubstrate is placed. For example, oxygen and helium are supplied to thplasma generating region, and a silane gas is supplied to the substrateusing a material gas inlet system D4. By this configuration, a siliconoxide film can be formed on the substrate.

[0073]FIG. 7 is a top view of another embodiment of the inventedsemiconductor thin film forming system. A substrate transfer chamber C7is respectively connected to a load-unload chamber C1, a plasma-enhancedCVD chamber C2, a substrate heating chamber C3, a hydrogen plasmatreatment chamber C4, and a laser irradiation chamber C5 via gate valvesGV1 through GV6. Laser beams are supplied from a first beam line L1 anda second beam line L2 and are applied to the substrate surface via alaser composing optical system opt1, a laser irradiation optical systemopt2, and a laser inlet window W1. Gas supply systems gas1 to gas7, andventilators vent1 to vent7 are connected to the individual processchambers and the transfer chamber. By this configuration, desired gasspecies can be supplied, and target process pressures can be set. Inaddition, the ventilation and degree of vacuum can be controlled.Substrates sub2 and sub6 to be processed are placed horizontally asindicated by dotted lines in the figure.

[0074]FIG. 8 shows process flow charts showing an application of theinvented semiconductor thin film forming system to a production processof thin film transistors. The process includes the following steps.

[0075] (a) A glass substrate sub0 is cleaned to remove organicsubstance, metals, fine particles and other impurities. Onto the cleanedglass substrate, a substrate covering film T1 and a silicon tin film T2are sequentially formed. As the substrate covering film, a silicon oxidefilm is formed to a thickness of 1 μm by low pressure vapor deposition(LPCVD) process at 450° C. with silane and oxygen gases as materials. Byusing the LPCVD process, the overall exterior surface of the substratecan be covered with a film, except for a region where the substrate isheld (this embodiment is not shown in the figure). Alternatively, theprocess can employ, for example, a plasma-enhanced CVD process usingtetraethoxysilane (TEOS) and oxygen as mat rials, a normal pressure CVDprocess using TEOS and ozone as materials, or the plasma-enhanced CVDprocess shown in FIG. 17. An effective substrate covering film includessuch a material as to prevent the diffusion of impurities in thesubstrate material. Such impurities adversely affect semiconductorelements. The substrate may comprise, for example, a glass having aminimized alkali metal concentration or a quartz or glass having apolished surface. The silicon thin film is formed to a thickness of 75nm by LPCVD at 600° C. with a disilane gas as a material. Under theseconditions, the resulting film is to have a hydrogen atom concentrationof 1 atomic percent or less, and the film can be prevented from, forexample, roughening due to emission of hydrogen in the laser irradiationprocess. Alternatively, the plasma-enhanced CVD process shown in FIG. 17or a conventional plasma-enhanced CVD process can be employed. In thiscase, a silicon thin film having a low hydrogen atom concentration canbe obtained by adjusting the substrate temperature or the flow rateratio of hydrogen to silane or the flow rate ratio of hydrogen tosilicon tetrafluoride.

[0076] (b) The substrate prepared in Step (a) is subjected to a cleaningprocess to remove organic substances, metals, fine particles, surfaceoxide films and other unnecessary matters. The cleaned substrate is thenintroduced into the invented thin film forming system. The substrate isirradiated with a laser beam L0 to convert the silicon thin film to acrystallized silicon thin film T2′. The laser-induced crystallization isperformed in a high purity nitrogen atmosphere of 99.9999% or more at apressure of 700 Torr or more.

[0077] (c) After the completion of Step B, the process chamber isevacuated, and the substrate is then transferred via a substratetransfer chamber to a plasma-enhanced CVD chamber. As a first gateinsulating film T3, a silicon oxide film is deposited to a thickness of10 nm at a substrate temperature of 350° C. from material silane,helium, and oxygen gases. Where necessary, the substrate is thensubjected to hydrogen plasma treatment or to heating and annealing.Steps (a) to (c) are conducted in the invented thin film forming system.

[0078] (d) Islands composed of laminated silicon thin film and siliconoxide film are then formed. In this step, the etching rate of thesilicon oxide film should be preferably higher than that of the siliconthin film according to etching conditions. By forming a stepped ortapered pattern section as illustrated in the figure, the gate leak isprevented, and a thin film transistor having a high reliability can beobtained.

[0079] (e) The substrate is then cleaned to remove organic substances,metals, fine particles and other impurities, and a second gateinsulating film T4 is formed to cover the above-prepared islands. Inthis example, a silicon oxide film 30 nm thick is formed by the LPCVDprocess at 450° C. from material silane and oxygen gases. Alternatively,the process can employ, for example, the plasma-enhanced CVD processusing tetraethoxysilane (TEOS) and oxygen as materials, the normalpressure CVD process using TEOS and ozone as materials, or theplasma-enhanced CVD process as shown in FIG. 18. Next, an n⁺ siliconfilm 80 nm thick and a tungsten suicide film 110 nm thick are formed asgate electrodes. The n⁺ silicon film should be preferably aphosphorus-doped crystalline silicon film formed by the plasma-enhancedCVD processor LPCVD process. The work is then subjected tophotolithography and etching processes to yield a patterned gateelectrode T5.

[0080] (f1, f2) A doping region T6 or T6′ is then formed using the gateas a mask. When a complementary metal oxide semiconductor (CMOS) circuitis prepared, an n channel TFT requiring an n⁺ region, and a p⁻ channelTFT requiring a p⁺ region are separately formed. The doping techniqueincludes, for example, ion doping where injected dopant ions are notsubjected to mass separation, ion injection, plasma-enhanced doping, andlaser-enhanced doping. According to the application of the product or thused technique for doping, the surface silicon oxide film is remained asintact or is removed prior to doping (f1, f2).

[0081] (g9, g2) An interlayer insulating film T7 or T7′ is deposited,and a contact hole is formed, and a metal is deposited thereon. The workis then subjected to photolithography and etching to yield a metallicwiring T8. Such interlayer insulating films include, but are not limitedto, a TEOS-based oxide film, a silica coating film, and an organiccoating film that can provide a flat film. The contact hole can beformed by photolithography and etching with a metal. Such metals includelow resistant aluminium, copper, and alloys made from these metals, aswell as tungsten, molybdenum, and other refractory metals. The processincluding these steps can produce a thin film transistor having highperformances and reliability.

[0082]FIG. 9 illustrates an embodiment where an alignment mark ispreviously formed and laser irradiation is performed with reference tothe alignment mark. FIG. 10 illustrates another embodiment where analignment mark is formed concurrently with laser irradiation. Theseembodiments are based on the TFT manufacture process flow, and arebasically similar to the process shown in FIG. 8. The distinguishablepoints of these embodiments are described below.

[0083] In FIG. 9(a), a glass substrate sub0 is cleaned to remove organicsubstances, metals, fine particles, and other undesired matters. On thecleaned substrate, a substrate covering film T1 and a tungsten silicidefilm are sequentially formed. The work is then patterned byphotolithography and etching to form an alignment mark T9 on thesubstrate. A mark protective film T10 is formed to protect the alignmentmark, and a silicon thin film is then formed.

[0084] In FIG. 9(b), upon laser light exposure, a target region isexposed to light with reference to the alignment mark. The alignment inthe successive step can be performed with reference to the preformedalignment mark or to an alignment mark formed by crystallized siliconthin film patterning (not shown).

[0085] In FIG. 10(b), a crystallized alignment mark T9′ is formedconcurrently with laser irradiation to the silicon thin film. Thecrystallized alignment mark is formed by utilizing a difference inmodification between an exposed region and a nonexposed region.

[0086] In FIG. 10(d), alignment in the photolithography process isperformed by using the crystallized alignment mark T9′. The work is thensubjected to an etching process to form islands composed of laminatedsilicon thin film and silicon oxide film.

[0087] The description has thus been made for the embodiment of theoptical source utilizing the eximer laser such as XeCl, KrF, XeF, ArF orthe like, however, various other kinds of laser such as YAG laser,carbon dioxide laser, or the semiconductor laser with the pulse emissioncan be used. The embodiment is applicable not only to the siliconsemiconductor thin film but also to the formation of the crystal thinfilm and the forming apparatus therefor.

[0088] Industrial Applicability

[0089] According to the present invention, in the case where theirradiation intensity is increased so as to obtain the crystallizedstructure of a better quality, it is possible to prevent the crystalbecoming microcrystalline or amorphous. Therefore, a technique isprovided for forming the silicon thin film with a small trap statedensity by the energy beam irradiation such as optical irradiation. Atechnique is also provided for applying the technique on the substratewith a large surface and a semiconductor device therefor. A device formanufacturing the electric field effective transistor utilizing siliconof a good quality, i.e. with an efficient characteristic can beprovided.

[0090] In the crystallization utilizing the fine beam which iscontrolled to the micron order, a crystal growth length by a single unitpulse has been increased twice as the conventional technique.

1. A thin film processing method of processing a thin film byirradiating an optical beam onto a thin film, wherein: one unit ofirradiation of the optical beam includes irradiations due to first andsecond optical pulses onto the thin film, the thin film is processed byrepeating the one unit of irradiation, and each of the first and thesecond optical pulse has different pulse waveforms.
 2. A thin filmprocessing method as claimed in claim 1, wherein: the one unit ofirradiation includes the the first optical pulse irradiation to the thinfilm and the second optical pulse irradiation to the thin film whichsubstantially starts simultaneously with the irradiation of the firstpulse.
 3. A thin film processing method as claimed in claim 2, wherein:a relationship between the first and the second pulse satisfies that apulse width of the first optical pulse is smaller than a pulse width ofthe second optical pulse and an irradiation intensity of the firstoptical pulse is not lower than an irradiation intensity of the secondoptical pulse.
 4. A thin film processing method as claimed in claim 1,wherein: the one unit of irradiation includes the first optical pulseirradiation to the thin film and the second optical pulse irradiation tothe thin film which substantially starts with a delay to the firstoptical pulse irradiation.
 5. A thin film processing method as claimedin claim 4, wherein a relationship between the first and the secondpulse satisfies (a pulse width of the first optical pulse)<(a pulsewidth of the second optical pulse).
 6. A thin film processing method asclaimed in claim 5, wherein the relationship between the first and thesecond pulse satisfies (the irradiation intensity of the first opticalpulse)≧(the irradiation intensity of the second optical pulse).
 7. Athin film processing apparatus for processing a thin film by irradiatingan optical beam onto the thin film, comprising: first and a second pulseoptical sources for producing first and second optical pulses which havepulse waveforms different to each other, one unit of irradiation of theoptical beam includes irradiations due to first and second opticalpulses onto the thin film, and the thin film is processed by repeatingthe one unit of irradiation.
 8. A thin film processing apparatus asclaimed in claim 7, wherein: the one unit of irradiation includes thethe first optical pulse irradiation to the thin film and the secondoptical pulse irradiation to the thin film which substantially startssimultaneously with the irradiation of the first pulse.
 9. A thin filmprocessing apparatus as claimed in claim 8, wherein: a relationshipbetween the first and the second pulse satisfies that a pulse width ofthe first optical pulse is smaller than a pulse width of the secondoptical pulse and an irradiation intensity of the first optical pulse isnot lower than an irradiation intensity of the second optical pulse. 10.A thin film processing apparatus as claimed in claim 7, wherein: the oneunit of irradiation includes the first optical pulse irradiation to thethin film and the second optical pulse irradiation to the thin filmwhich substantially starts with a delay to the first optical pulseirradiation.
 11. A thin film processing apparatus as claimed in claim10, wherein a relationship between the first and the second pulsesatisfies (a pulse width of the first optical pulse)<(a pulse width ofthe second optical pulse).
 12. A thin film processing apparatus asclaimed in claim 11, wherein the relationship between the first and thesecond pulse satisfies (the irradiation intensity of the first opticalpulse)≧(the irradiation intensity of the second optical pulse).